Imaging using MRI systems in the past required that data acquisition be substantially completed prior to image reconstruction. The reconstruction of the images in MRI systems is accomplished by transforming the data acquired using two-dimensional Fourier transforms. The data acquisition scan in the past proceeded from an edge of a two-dimensional Fourier transform plane through the center of the plane to an opposite edge. Most of the data is actually acquired around the center of the Fourier transform plane. Thus, in the past, it was necessary for the MRI system to first acquire at least more than half of the data prior to processing the data to provide the image.
Subsequently, apparatus and methods were promulgated, see for example, U.S. Pat. No. 4,888,552 referred to hereinabove, wherein the acquisition proceeds from the center or near the center of the Fourier transform plane outwardly enabling the almost immediate processing of the data to provide the image.
However, the acquisition from the center outwards often causes artifacts, especially Gibbs artifacts. One of the reasons for the Gibbs artifacts in the evolving images is that the sampling time is extremely limited. See, for example, U.S. Pat. No. 4,950,991, which issued on Aug. 21, 1990, and is based on Israeli Patent Application 86570 filed on May 31, 1989, and the Israeli Patent Application 098053 filed on May 3, 1991, both assigned to the Assignee of this Application. The contents of the patent and the later filed application are hereby incorporated herein.
A signal representation in the image domain near a discontinuity, for example, includes an "oscillatory overshoot" which is approximately 9% of the magnitude of the signal at the discontinuity. When few sampling points are available; i.e., in the early phases of the evolving image, the 9% amplitude is approximately in the center of the discontinuity. As more sampling points are taken the overshoot is compressed towards the edge of the discontinuity which reduces the effects of the artifact to improve the spatial resolution.
Accordingly, during evolving image processing, when few sampling points are available, the Gibbs artifact is especially pronounced making the early images of the evolving image virtually illegible for diagnostic purposes.
Data acquisition requires that a patient being imaged be placed in a strong static magnetic field. The strong static magnetic field aligns the protons of certain elements within the body. The aligned protons are exposed to RF pulses which nutate the aligned protons into a plane transverse to the static magnetic field when the frequency of the RF pulses is the Larmor frequency.
After the application of the RF pulses, the nutated protons rotate in the transverse plane and also dephase and tend to realign themselves with the magnetic field. The signals due to the movement of the nutated protons in the transverse plane generally are known as free induction decay (FID) signals which are proportional to the density of the nutated protons. These signals are used as raw data for reconstructing images or else echo procedures are used for rephasing the FID signals periodically to obtain what are known as echo signals. The echo signals are then used as raw data for reconstructing the images. The raw data is preferably transformed using two-dimensional Fourier transforms to obtain data for image pixels which correspond to locations in the patient.
The locations of the detected FID signals (herein "FID signals" include either FID signals themselves or the echo signals) are obtained in a well known manner using gradient pulses.
In the prior art, in general, image reconstruction does not commence until at least half of the phase encoding gradient pulses have been applied. In the past, in general, the first phase encoding gradient pulse applied was the maximum negative phase encoding gradient pulse followed by the maximum negative plus one phase encoding gradient pulse and sequentially through to the maximum positive phase encoding gradient pulse.
As is well known, at the zero value of a phase encoding gradient pulses, the received signal is generally largest; i.e., most of the data is acquired. That is why image reconstruction could not start until at least the zero phase encoding gradient was applied; i.e., until at least half the phase encoding gradients were applied. Where only half of the phase encoding gradients were used, computations were necessary to construct the other half of the data. As is recognized in the U.S. Pat. No. 4,721,912 and in the '552 Patent referred to above, if data reconstruction could be accomplished on line; i.e., while the data acquisition was in process; then, valuable throughput time would be saved. Where the image is reconstructed on-the-fly as in the '552 and the '912 patents, the clinician does not have to wait for complete image data to determine the efficacy of the imaging procedure being used. If the image location is wrong, for example, the clinician can restart the procedure and move the selected slice early in the data acquisition process without having to wait for the entire data acquisition. Time-saving is also effected by not having to wait for all the data to be acquired before reconstruction of the image starts, because as the data acquisition proceeds, the image becomes progressively clearer. Thus, when the physician obtains sufficient information, usually prior to the acquisition of all the scheduled data, he can stop the data acquisition process.
The evolving images early in the evolving image process, have low resolution, but have a high signal-to-noise ratio; thereby, theoretically compensating for the low resolution. However, such evolving images are adversely affected by the Gibbs artifacts to the point of demeaning the usefulness of the partial images for diagnostic purposes. The '552 Patent improved on the '912 Patent by, among other things, starting acquisition away from the zero point during the initial system instability; then after a fixed number of encoding gradients, during which the system stabilized, acquiring data at the zero point. However, because of the paucity of data, the Gibbs artifacts still remained and were pronounced. Notwithstanding the improved signal-to-noise ratio of the evolving images, the Gibbs artifacts degraded the evolving image to an extent wherein the evolving image procedure in many cases was hardly useful.
Accordingly, it is an object of the present invention to further improve on the "evolving image" procedure by reducing the artifacts through asymmetrical sampling, complex conjugating and filtering of the acquired signal which is particularly effective vis-vis the Gibb's artifact. Without the Gibbs artifact, the improvement in the legibility of the image during the evolving image process is readily apparent, Thus, without the Gibbs artifact, the relatively high SNR of the early evolving images provides a good useful legible image in pereceptibly less acquisition time.
A related object of the present invention is a reduction of the Gibb's artifact without adversely affecting the resolution, the SNR or the scan time of evolving images.
Yet another related object of the present invention is to provide methods and systems for acquiring usable relatively Gibbs artifact-free images on the fly in MRI systems; i.e., to provide MRI systems wherein image reconstruction commences almost simultaneously with image acquisition and nonetheless Gibbs artifacts are greatly minimized. Thereby maximizing the legibility and usefulness of even the early evolving images.